Wireless Internet services have been considered as an opportunity for a new market creation. At present, wireless Internet services have provided downloadable animated color graphics, called characters, and ring tones for mobile communication services.
However, these services do not completely satisfy various customer needs. That is, while requirements for low cost services and high-speed Internet service have increased, communication performance in a global system for mobile communication (GSM) and a code division multiple access (CDMA) do not fully satisfy customer needs. Therefore, it is required to develop a new portable Internet service. Specifically, it is required to provide a time division duplexing system in order to provide a low cost and high quality mobile multimedia service, and an orthogonal frequency division multiplexing (OFDM) method for efficiently reducing an inter-symbol interference (ISI), the OFDM method being supported by a simplified receiver and a cyclic prefix (CP) and used as a modulation method.
In the OFDM method, an input symbol sequence is parallelized and modulated into a plurality of subcarriers having orthogonality to each other. Compared to a conventional single carrier transmission method, the ISI may be efficiently reduced since a symbol transmission is maintained and a symbol duration is increased by the number of subchannels.
However, a frequency offset may occur since a transmitted frequency is not synchronized with a received frequency when a receiver tuning is inappropriate due to channel characteristics. The frequency offset changes a phase of the received signal and reduces decoding performance in a system. Especially in the OFDM method, since a symbol detection is performed for each subchannel, the orthogonality is not maintained between the respective subcarrier frequencies, and interference occurs between neighboring subchannels when the frequency offset occurs. In addition, since the respective subcarriers are densely distributed in a predetermined bandwidth as the number of OFDM subchannels is increased, the interference is significantly generated even when less frequency offset occurs.
Accordingly, methods for estimating and eliminating the offset generated in the receiver have been suggested. In the prior art, data-aided (DA) algorithms usually having synchronization information and non-data-aided (NDA) feedforward estimation structures having statistics on the received signals have been used.
In addition, because of the multipath channel, the signals received in the receiver of a portable Internet system are affected by the ISI before a fast Fourier transform (FFT) operation on the signals is performed. However, since the ISI is not considered in the prior art, the timing error and the frequency offset have not been correctly estimated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.